Implantable medical devices are used to treat patients suffering from a variety of conditions. Examples of implantable medical devices include implantable pacemakers and implantable cardioverter-defibrillators (ICDs), which are electronic medical devices that monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers as necessary. Pacemakers deliver relatively low-voltage pacing pulses in one or more heart chambers. ICDs can deliver high-voltage cardioversion and defibrillation shocks in addition to low-voltage pacing pulses
Pacemakers and ICDs generally include pulse generating circuitry required for delivering pacing and/or cardioversion and defibrillation pulses, control circuitry, telemetry circuitry, and other circuitry that require an energy source, e.g. at least one battery. In addition to a battery, ICDs include at least one high-voltage capacitor for use in generating high-voltage cardioversion and defibrillation pulses. Implantable medical devices (IMDs), including pacemakers, ICDs, drug pumps, neurostimulators, physiological monitors such as hemodynamic monitors or ECG monitors, typically require at least one battery to power the various components and circuitry used for performing the device functions.
IMDs are preferably designed with a minimal size and mass to minimize patient discomfort and prevent tissue erosion at the implant site. Batteries and capacitors, referred to collectively herein as “electrochemical cells,” contribute substantially to the overall size and mass of an IMD. Electrochemical cells used in IMDs are provided with a hermetically-sealed encasement for housing an electrode assembly, including an anode and cathode separated by a separator material, an electrolyte, and other components such as electrode connector feed-throughs and lead wires. The encasement includes a case and a cover that are sealed after assembling the cell components within the case.
The total amount of the anode and cathode material required in the cell will depend on the energy density, volume, voltage, current, energy output, and other requirements of the cell for a particular application. Anode and cathode material, with an intervening separator, may be arranged in a coiled electrode assembly. Both round and flat cylindrical coiled electrode assemblies are known in the art. Flat electrochemical cell designs tend to improve the volumetric efficiency of the cell because they are generally better suited for fitting within an IMD housing with other device components. Flat electrochemical cell designs may utilize a stacked electrode assembly wherein anode, cathode and intervening separator material are arranged in a stacked configuration. In stacked configurations, separate electrode layers or plates, which are often generally rectangular in shape and aligned in a stack, need to be interconnected. While the stacked configurations may improve volumetric efficiency, multiple interconnections can add to the complexity of cell assembly. The use of multiple, separate electrode layers can also contribute to tolerance stack-up, which may alter the balance of the anode and cathode material available and add undesired mass to the cell.
A serpentine anode design addresses some of these limitations in that a serpentine folded anode generally requires fewer interconnections and fewer piece parts reducing tolerance stack-up. For example, a lithium foil anode may be wrapped through cathode layers in a serpentine fashion. Since inner wraps face cathode material on both sides, a thicker piece of lithium foil is generally used for the inner wraps. The outermost wraps face cathode material on one side requiring a thinner lithium foil in a lithium-balance battery. The serpentine anode material will face opposing cathode material on both sides requiring a sheet of anode material on both sides of a collector. As such, a serpentine anode formed of lithium foil for use in a battery generally requires at least three pieces of lithium in two different thicknesses.
As it is desirable to minimize overall IMD size, electrochemical cell designs that allow cell size and mass to be reduced are desirable. Reduction of electrochemical cell size may allow balanced addition of volume to other IMD components, thereby increasing device longevity and/or increasing device functionality. Other electrochemical cell design considerations motivating new cell designs include reducing manufacturing cost, time and complexity.